Two separate ramp surfaces formed by the cam means are generally provided in such machines for the purpose of respectively displacing the needles in a forward stroke (hereinafter assumed to be upward) and in a return stroke (hereinafter assumed to be donward). The first ramp surface elevates the needles in the first part of a stitch-forming cycle to let their hooks engage a new length of thread to be drawn into a loop during the subsequent downstroke controlled by the second ramp surface. This downstroke should be as fast as possible, not only in order to shorten the overall stitch-forming cycle but also to provide a more effective staggering between the pull-down operations of adjoining needles simultaneously engaging the same thread for the purpose of reducing the number of friction points between the thread and these needles. On the other hand, such an acceleration of the downstroke in a conventional knitting machine would require an increase in the slope of the second or needle-lowering ramp surface with resulting augmentation of the shear stresses exerted by the cam upon each needle.